This invention relates to a rheology control additive for high solids coating compositions.
Conventional coating compositions of a high molecular weight acrylic polymer and melamine crosslinking resins are well known in the art as shown by Vasta U.S. Pat. No. 3,622,651, issued Nov. 23, 1971; Hick U.S. Pat. No. 3,841,895, issued Oct. 15, 1974; Parker U.S. Pat. No. 3,674,734, issued July 4, 1972 and Parker U.S. Pat. No. 3,637,546, issued Jan. 25, 1972. These patents illustrate high quality coating compositions but have a relatively high solvent content to provide for good application properties and good properties of the resulting dried finish. To utilize these compositions in areas which have strict air pollution regulations, pollution abatement equipment is required. This equipment is expensive and increases capital investment of a plant and is costly to operate. Any attempt to reduce the solvent content of these conventional compositions generally results in finishes which have either a poor appearance or unacceptable properties or both.
In an effort to obtain high solids coating compositions that can be applied by conventional spraying techniques, the molecular weight of the polymer used in these compositions was reduced substantially and solvent content of the coating compositions was reduced. This reduction in molecular weight caused a problem of poor appearance, sagging of the finish on application, pulling away from edges after application and cratering of the finish. When aluminum flake pigments were used in these composition, poor appearance was caused by improper orientation of the aluminum flake in the finish.
There is a great need for a rheology control additive for high solids coating compositions that provides a reduced viscosity under typical spraying conditions when the coating composition is applied and that provides a substantially increased viscosity after application which prevents sagging and cratering of the resulting finish. Also, the additive should reduce pulling away of the finish from edges of a substrate to which the finish has been applied and should provide proper orientation of metallic flakes used in the coating. High solids coating compositions containing such an additive will have an overall appearance that is acceptable particularly for exterior finishes of automobiles and trucks.